Please help me with my SU carb woes....

Ian_Fearn · Post by **Ian_Fearn** » Sun Sep 27, 2009 7:09 pm

Hello all, first post on this forum although i've been reading some damn useful info on here for the past couple of months.

I've got a problem with my 85 Range Rover 3.5 V8. I recently junked the old Strombergs in favour of SU's. The SU's were apparently off a low mileage LDV ambulance.

I had a problem with the revs holding but i've replaced the butterflies with solid ones which seems to have cured that.

The problem I now have since changing the butterflies is a reluctance to rev in those initial 2-300 rpms from idle. Pulling away means revs at 2000 otherwise it stalls. Over this it runs great.

The replacement solid butterflies were slightly bigger than the originals hence they dont sit square in the body like the old ones. I've set them as central as possible. Does anybody know if replacement butterflies are bigger or have I been sent the wrong ones??

Being larger it means the vacuum orifice now sits behind the butterfly at idle rather than in front. Would this make any difference??

I also appear to be getting a few drops of fuel from the overflow pipes, normally after the engine has been switched off.

Mixture seems good, checked with a colourtune and the balance seems acceptable too. Timing was checked prior to the butterfly swap and is correct too.

RoverP6B · Post by **RoverP6B** » Sun Sep 27, 2009 10:16 pm

Hello Ian,

What type of SU carburettors have you fitted? Are they HIF6?

The butterflies whether fitted with poppet valves or not should be exactly the same in diameter. You will see that they have a chamfer and as such can only be fitted correctly one way.

Hesitation from off idle is an indication of too lean a mixture. That does not mean that the idle mixture is incorrect. Adjusting the mixture must only ever be done at idle, and from there the taper of the needle is responsible for setting the mixture.

Have you changed the needles or are they the ones that were in the carburettors when you purchased them?

The drops of fuel coming from the overflow pipes after a run is normal and not an indication of an impending problem. Fuel vapour travels from the float bowls down the overflow tubes and condenses at the outlets, forming the droplets that you see.

Should fuel at any time actually flow from the overflow tubes then that is a different story.

Ron.

ChrisJC · Post by **ChrisJC** » Mon Sep 28, 2009 7:17 am

Ron's covered pretty much everything.

You say they've been balanced, the other area to check is that they both open together, i.e. the slack in the linkage between the carbs doesn't mean that one opens quite a bit before the other. Mind you, under these circumstances, it's basically running on four cylinders when you lightly touch the throttle, so it's pretty lumpy!

Chris.

Ian_Fearn · Post by **Ian_Fearn** » Mon Sep 28, 2009 10:59 am

Thanks for the replies.

I rang Burlen (who supplied the butterflies) who could only think that i've fitted the discs the wrong way round as suggested by Ron (RoverP6B).

They confirmed the discs should be the same size so perhaps I really have fitted them the wrong way round? Bit of a school boy error if I have but i'll find out tonight.

ChrisJC · Post by **ChrisJC** » Mon Sep 28, 2009 12:58 pm

They must be the right size thinking about it, otherwise they wouldn't fit down the bore in the carb.

Maybe you have just got them 180deg flipped.

Chris.

Ian_Fearn · Post by **Ian_Fearn** » Mon Sep 28, 2009 5:02 pm

Just driven it home from work and stripped one of the carbs off. I can confirm the taper is the same way round as the one i removed. A simple trial showed fitting it the other way round was totally wrong. The new one has a far more pronounced taper than the original.

Comparing the old and new shows the new one to be oval whereas the original is circular. In the horizontal plain the diameter is the same but the vertical plain is approx 1mm greater on the new one.

The guy at Burlen claimed they 'should' be the same. He confirmed they'd sent the right bits too.

The issue I believe I have is that at idle with the original butterflies the vacuum advance hole is to the atmosphere side of the butterfly but on the replacement the hole is to the engine side (presumably greater pressure??).

Burlen didnt believe the butterfly had been altered from the original spec other than the poppet valve deletion.

CastleMGBV8 · Post by **CastleMGBV8** » Mon Sep 28, 2009 6:37 pm

The old trick is to solder up the poppet valves in the original throttle discs and refit them.

Kevin.

Ian_Fearn · Post by **Ian_Fearn** » Mon Sep 28, 2009 7:13 pm

I think this is what i'll end up doing. I was hoping the replacement butterflies would just be fit and forget but as with lots of things its never as simple as it first seems!

CastleMGBV8 · Post by **CastleMGBV8** » Mon Sep 28, 2009 7:24 pm

The replacements would appear to be faulty if thay are not round so return them and ask for a refund.

kevin.

crayefish · Post by **crayefish** » Mon Sep 28, 2009 7:45 pm

CastleMGBV8 wrote:The replacements would appear to be faulty if thay are not round so return them and ask for a refund.

kevin.

well butterflies shouldnt be exactly round if they are to fit into the carb bore at an angle... but pretty close to round mind

Ian_Fearn · Post by **Ian_Fearn** » Mon Sep 28, 2009 7:56 pm

In all your expert opinions would the positioning of the hole for the vacuum advance many any difference based on my previous comments??

topcatcustom · Post by **topcatcustom** » Tue Sep 29, 2009 7:18 am

If you solder up the holes make sure to do it well! I'd be nervous of the lump of tin coming out one day and having fun bouncing around in various cylinders...

RoverP6B · Post by **RoverP6B** » Tue Sep 29, 2009 10:03 am

The vacuum advance take off point on the SU carburettor (HIF6) is exposed to normal atmospheric pressure when the engine is at idle. The butterfly is closed and the piston is sitting against the bridge.

This ensures that no vacuum advance is provided at idle.

If the butterflies that you have fitted now cover the vacuum advance take off point, this will allow vacuum advance to occur at idle. This is totally wrong.

The butterflies that you have purchased are not correct, in fact I have never heard of such a botch!

You will be well advised to return them and obtain a correctly made pair.

Your engine will not run as it was originally designed to until you do.

Ron.

kiwicar · Post by **kiwicar** » Wed Sep 30, 2009 6:53 am

For information there are about 6 different butterflys for the 1 3/4 su that I know of, all are different ellipses depending on the vesion of the carb they are designed for, there is a little ally tag somewhere on the carb body to tell you which version of the su you have. the different butterflys are so the carb can have different angles between fully open and fully closed. you will find that some SUs open "backwards" you can convert them to "normal" opperation, but is is a bit of a hassel. You have already found out they uncover the vaceum taping for the distributor at different points so you need to check this. some versions of the carbhave transition passages and funny drillings to blead extra air past the main jet to try and get them to pass emisions (really only on the last applications) so check for these. They are a great carb shame BL had to bggr about with them!
Mike

Ian_Fearn · Post by **Ian_Fearn** » Wed Sep 30, 2009 11:37 am

Can anybody recommend a supplier??

Burlen claim to supply only 1 type!